1. Field of Invention
This invention relates to economical methods and apparatuses for manufacturing a high-pressure die casting.
2. Prior Art
In a conventional die-casting method, a mold or die typically includes at least two mating parts that can be separated to facilitate removal of the cast part when it has cooled sufficiently. Only when the mold is closed could the molten metal be poured into the sleeve of a die casting machine. The molten metal is then forcibly injected into the mold cavity. For the purpose of simplicity, the mold or die will be described herein with reference to two mating parts or halves. Until the metal is cool enough to be removed, the mold halves must be held together under pressure by a clamping force. The clamping force is often extremely high in order to overcome the force exerted by the molten metal as it is driven into the cavity and thereby keep the die halves effectively sealed.
It is known that the quality of the casting improves with more rapid flow of the molten metal into the die cavity. With conventional control systems, however, as the velocity of the flow of the molten metal into the cavity increases, the impact pressure that the metal exerts on the die increases. At some point, that pressure might be sufficiently high to overcome the clamping force, causing the die halves to separate and allow metal to leak from the cavity, thereby destroying the integrity of the cast part. As a result, the pressure within the die cavity must be controlled in the die casting process. On other hand, the more rapid the flow of the molten metal into the die cavity, the more gases can be entrapped in the casting after the completion of the injection.
To reduce the pressure that the metal exerts on the die, a method is shown on U.S. Pat. No. 7,174,947. This method reduces the impact pressure of the molten metal on the mold by appropriately setting the volume of the overflow chambers and the cross-sectional area of the gates of the overflow chambers.
One way to reduce gases in die-casting is with the vacuum die-casting method. However, the internal gas pressure of the cavity has a limited vacuum range from 100 to 500 millibar (see U.S. Pat. No. 6,648,054) even this system equip with an expensive vacuum unit. Therefore, we need a new method to further reduce gases.
The second group of improved die casting methods has proposed for the manufacture of wieldable die-cast products: controlling flow rate at the gate. An example is the Japanese Patent Laid-Open Publication No. HEI-4-172166 entitled “METHOD OF MANUFACTURING ALUMINUM CAST PARTS FOR BRAZING”. According to this method, as shown in the drawing figures of the publication, the flow rate of molten metal at a gate (gate velocity) is switched stepwise between a low flow rate ranging from 0.3 msec to 0.6 msec for a first half of processing and a high flow rate ranging from 10 msec to 30 msec for a latter half of the processing.
Koya, et al on U.S. Pat. No. 6,352,099 uses a new method to achieve the above purpose, according to this invention, and provides an aluminum alloy die casting method comprising of the steps of providing a die casting machine having a gate for the passage of molten aluminum alloy, setting a flow rate of the molten aluminum alloy at the gate to be in a range of 5 msec to 15 msec, and press-injecting the molten aluminum alloy into a cavity of a die.
However, for most of the die casting process, if we need to achieve good surface finishing, the typical flow rate of molten metal at a gate is 30 msec for Al, 45 m/s for Zn, and 65-90 msec for Mg (Walkgton, William. Surface Defects: Guide to Correcting the Problem. 517. Rosemont, Ill.: North American Die Casting Association, 2005.). Therefore a better method is needed to keep a higher flow rate and reduce entrapped gases.
The third method shown is on U.S. Pat. No. 5,718,280, which uses special units to inhibit a molten metal from involving gases contained in a plunger sleeve. However, the die casting apparatus disclosed in the publication has a complicated construction, because it requires a plunger chip movably disposed into a plunger sleeve and an electromagnetic induction coil disposed around the plunger sleeve. These two addition units will also cause more maintenance work during liquid metal is poured outside the pour hole.